Observation: Sherlock is wearing his bed sheet.
Deduction: Sherlock sleeps naked.
Posit: Sherlock is only wearing the sheet to Skype. He usually wears nothing around the house.
Sherlock walks around the house wearing only a sheet, potentially maybe even less.
Sherlock often doesn’t seem to notice whether John is actually around or not.
If you were prone to bouts of partial or full nudity that you didn’t want anyone to witness, you’d pay more attention to the location of your “flatmate” to prevent embarrassing mishaps.
I infer from this that Sherlock walks around in this state whether John is in the room or not.